Curing compositions that cure on application of active energy rays, such as ultraviolet rays or electron beams, or heat energy, are broadly used on account of their advantages over solution type curing compositions which are hardened by removing solvents in terms of resources saving, non-environmental pollution, rapid curing, and the like advantages.
In general, active energy ray-curing compositions mainly comprise a (meth)acrylate type prepolymer, such as polyester (meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate or polyether (meth)acrylate; or a resin or prepolymer of an unsaturated polyester, etc., and further comprise a reactive diluent, such as a (meth)acrylate having one (meth)acryloyl group (e.g., phenoxyethyl (meth)acrylate or carbitol (meth)acrylate) or a polyol poly(meth)acrylate (e.g., trimethylolpropane tri(meth)acrylate or tripropylene glycol di(meth)acrylate). The viscosity, curability, and adhesiveness of the active energy ray-curing compositions can be improved by appropriately selecting the reactive diluent. However, conventional active energy ray-curing compositions exhibit large volume shrinkage upon curing, and therefore adhesiveness to a substrate is insufficient. Thus, when the conventional compositions are used as an adhesive, materials that can be adhered are restricted.
Known compositions that cure on heating include those comprising an unsaturated polyester and styrene, as well as the above-mentioned (meth)acrylate type compositions. However, these compositions have a low curing rate and give off an offensive odor. When these compositions are used as an adhesive in particular, there are problems in that satisfactory shear strength is not obtained or the adhesive joint tends to be separated.
The present inventors have conducted extensive investigations for the purpose of developing a curing composition which provides a cured product excellent in hardness and adhesion and, when used as an adhesive in particular, provides an adhesive joint which has sufficient shear strength and undergoes no separation.
In the meantime, it is known that properties of wood, such as dimensional stability, resistance to water absorption, and wear resistance, can be improved by impregnating wood with a curing composition and then curing the composition. The resulting wood-plastic composite material called a wood-plastic combination (WPC) is utilized as various construction materials.
As the curing compositions for WPC, curing compositions comprising unsaturated polyesters dissolved in monomers such as styrene and methyl methacrylate are known. However, since such compositions have hydrophobic properties, there are problems in that they cannot be impregnated sufficiently into the inside of wood, failing to impart the described property. Further, the monomers remain uncured in impregnated wood due to their poor reactivity. It follows that the uncured monomers are incorporated into the wood, which gives rise to a problem of offensive odor upon cutting or polishing after curing.
In order to overcome the above problems, use of an aqueous emulsion of a hydrophobic (meth)acrylate type oligomer of lower volatility as an impregnation composition has been studied as described in JP-A-55-118804 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Examples of the hydrophobic (meth)acrylate type oligomer include (meth)acrylate compounds obtained by addition of phenyl glycidyl ether with (meth)acrylic acid, and (meth)acrylate type oligomer obtained by addition of a condensate of phthalic acid and diethylene glycol with (meth)acrylic acid. However, since the (meth)acrylate type oligomers described in JP-A-55-118804 have high hydrophobic properties, their impregnation into wood is not satisfactory.
An aqueous solution of a water-soluble (meth)acrylate type oligomer is also suggested in JP-A-4-82707 for the same purpose as the above aqueous emulsion of hydrophobic (meth)acrylate type oligomer. Examples of the water-soluble (meth)acrylate type oligomer include those obtained by copolymerizing a hydrophilic component such as an alkali metal salt of (meth)acrylic acid with other (meth)acrylic compounds. In this case, however, a cured product of the oligomer also has hydrophilic properties on account of the water-solubility of the oligomer, failing to impart water and moisture resistance to wood.
Hence, the present inventors have conduced extensive investigations in order to develop a curing composition for impregnation of wood which has low volatility and is easy to handle, exhibits excellent wood impregnation properties and high reactivity, and is therefore capable of imparting moisture resistance and dimensional stability to wood.
Throughout the specification and claims, the term "(meth)acrylic acid" means acrylic acid or methacrylic acid; the term "(meth)acrylate" means an acrylate or a methacrylate; and the term "(meth)acryloyl group" means an acryloyl group or a methacryloyl group.